1. Field of the Invention
This invention relates to circuit interrupters and more particularly to circuit interrupters employing surge-limiting resistors and to fluid blast circuit interrupters of the high speed type adaptable for multiple break operation.
2. Description of the Prior Art
Transmission networks are used by electric utility companies to carry electrical energy from the point where it is generated to the point where it is ultimately distributed to consumers. System voltages on these transmission networks have been increasing until voltages of 500,000 volts are now common, and voltages of 700,000 volts and above are currently in use or under study. These extremely high voltages present many problems in designing equipment for safely switching components of the transmission network. For example, when an unenergized transmission line is connected to a source of electrical energy at these extremely high voltages, a voltage surge is produced which can have a value of several times the system voltage. Similar surges can occur when an energized transmission line is disconnected from the source. Surges of this magnitude present a stress upon the insulation components of the extra high voltage transmission lines which is even greater than that produced by natural lightning. Thus, it is required that the insulation components be increased in size and capacity to protect against these switching surges. This increases the construction cost of the transmission line.
As is well known in the art, the magnitude of switching surges can be reduced by temporarily inserting a resistor in series with the circuit interrupter. A circuit breaker employing resistors to limit the magnitude of switching surges is described in U.S. Pat. No. 3,291,947 issued Dec. 13, 1966 to the applicant and assigned to the assignee of the present invention. This interrupter employed resistors mounted internally within the housing of the interrupter head. Although the resistors are in series in the electrical circuit for only a fraction of a second, the heat produced by passage of current therethrough is considerable. With the increase in system voltages as previously described, the heat generated by current through the surge limiting resistors is greatly increased. It would thus be desirable in certain applications to provide surge limiting resistance which is external to the interrupting head. It is also desirable to provide means for switching resistance into and out of the circuit to be interrupted at the proper time in the interruption sequence. Since the full available short circuit current will not pass through the resistor and associated switching means, it is desirable to provide cost effective switching means which take advantage of this reduced current while at the same time providing sufficient capability to withstand full system and surge voltages. In order to further reduce the cost of the switching means associated with a surge limiting resistor, it is desirable to provide a high degree of parts commonality with existing circuit interrupters.
In the U.S. Pat. No. 3,590,319, issued June 29, 1971 to Paul Baltensperger, there is disclosed apparatus to switch a surge-attenuating resistance. The apparatus disclosed in the above-mentioned patent is connected in series circuit relationship to the power interrupter. Thus, the contacts of the apparatus carry full current. It is desirable to provide a system in which the contacts of the resistor switch need only carry a reduced current for a limited time, allowing the use of smaller, lighter components resulting in quicker response due to lower mechanical inertia.